Switching power supply circuits are utilized in battery chargers, AC adapters, and the like, because they offer a stable power supply. Drive methods (switching methods) for switching devices can be classified into two main types: self-excited oscillation and externally-excited oscillation. In the self-excited oscillation type, positive feedback of a drive signal, which indicates a voltage generated in a feedback winding of an inductance device like a transformer, is executed. The drive signal is fedback to a control terminal of a switching device to cause an oscillation operation thereof.
As this type of self-excited switching power supply circuit, examples are known such as the circuit shown in FIG. 4 (for example, Japanese Patent Laid-Open Publication No. 2002-051546). Hereinafter, this conventional self-excited switching power supply circuit 100 will be explained with reference to FIGS. 4 to 6. In FIG. 4, a direct current power supply 1 is an unstable power supply with a fluctuating voltage. This direct current power supply 1 has a high voltage side terminal 1a and a low voltage side terminal 1b. A transformer 2 is configured from a primary winding 2a, a feedback winding 2b that is provided on the primary side, and a secondary output winding 2c. Further, a field effect transistor (hereinafter referred to as “FET”) 3 for oscillation is also provided. A start-up resistor 21 is utilized to apply a forward bias (in other words, a gate voltage equal or more than a threshold voltage VTH) to a gate of the FET 3 during circuit start-up. An electrical resistor 25, which is connected in-series with the start-up resistor 21, has a resistance that is small as compared to that of the start-up resistor 21. Accordingly, voltage of the direct current power supply 1 is divided such that the circuit does not start-up when a low direct current voltage is output.
Further, the self-excited switching power supply circuit 100 is also provided with: a Zener diode 6 that prevents excessive input to the gate; a feedback capacitor 12 which is connected in-series between the feedback winding 2b and the gate of the FET 3 and which, along with a feedback resistor 23, configures an ON-control circuit; an electrical resistor 24 that prevents excessive input to the gate; and an OFF-control transistor 5 that connects a collector to the gate, and an emitter to the low voltage side terminal 1b. A control resistor 22 configures an oscillation stabilization circuit along with an OFF-control capacitor 11. A junction of this control resistor 22 and the OFF-control capacitor 11 is connected to a base of the OFF-control transistor 5.
A rectifying diode 4 and a smoothing capacitor 13, which are provided on the secondary output winding 2c side, configure a rectifying smoothing circuit. The diode 4 and the smoothing capacitor 13 rectify and smooth an output of the secondary output winding 2c, and the output is then output between a high voltage side output line 20a and a low voltage side output line 20b. 
With the self-excited switching power supply circuit 100 configured as described above, a direct current voltage is applied to the high voltage side terminal 1a and the low voltage side terminal 1b of the power supply 1 to charge the feedback capacitor 12 via the start-up resistor 21 (in FIG. 4, the electrode toward the bottom is positive, and that toward the top is negative). Accordingly, the charge voltage of the feedback capacitor 12 rises gradually.
When the charge voltage of the feedback capacitor 12 reaches the threshold voltage VTH, positive bias voltage is applied to the gate of the FET 3, and the FET 3 is turned on (there is electrical continuity between the drain and the source).
Next, a self-excited oscillation operation executed after the FET 3 turns on will be explained with reference to FIGS. 5 and 6.
FIGS. 5 and 6 show respective operation waveforms for the sections indicated by reference numbers (1) to (6) in FIG. 4, when the conventional self-excited switching power supply circuit 100 is caused to execute self-excited oscillation under conditions in which the power supply voltage of the direct current power supply 1 is applied at 200V. The resistances of the start-up resistor 21 and the electrical resistor 25 are set respectively at 1.5 MΩ and 100 kΩ. The capacity of the feedback capacitor 12 and the resistance of the feedback resistor 23 are set respectively at 0.01 μF, and 100 Ω.
After the FET 3 is turned on, an exciting current starts to flow from the direct current power supply 1 to the primary winding 2a that is connected in-series therewith. An induced electromotive force is generated in each of the windings of the transformer 2 (refer to the voltage waveform of the feedback winding 2b indicated by the section between time t12 and time t10 in (5) of FIG. 6). Accordingly, excitation energy is stored in the transformer 2. At this time, the voltage that is generated in the feedback winding 2b as a drive signal charges the OFF-control capacitor 11 via the control resistor 22. Consequently, a base voltage of the OFF-control transistor 5 rises (refer to the section between time t12 and time t10 in (a) of FIG. 5).
Further, during an ON period of the FET 3 between time t12 and time t10, an induced voltage generated in the feedback winding 2b (refer to (5) of FIG. 6) is superimposed on a charge voltage (refer to (6) of FIG. 6) of the feedback capacitor 12, and thus the gate voltage of the FET 3 (refer to (2) of FIG. 6) is maintained at equal to or more than the threshold voltage VTH thereof. At this time, the Zener diode 6 prevents excessive input to the gate.
When the OFF-control capacitor 11 is charged, and the charge voltage thereof (the base voltage of the OFF-control transistor 5) has become equal to or more than a bias voltage (refer to time t10 in (a) of FIG. 5), a base current flows in the OFF-control transistor 5. Therefore, electrical continuity is established between the collector and emitter. Accordingly, because of the OFF-control transistor 5, the gate of the FET 3 is effectively short circuited by the low voltage side terminal 1b and the FET 3 is turned off (refer to (b) of FIG. 5 and (2) of FIG. 6).
When the FET 3 is turned off in this way, the current flowing in the transformer 2 is effectively interrupted, and a so-called flyback voltage (an induced counter-electromotive force) is generated in the windings (refer to the section between time t10 and time t11 in (d) of FIG. 5). At this time, the flyback voltage generated in the secondary output winding 2c is rectified and smoothed by the rectifying smoothing circuit formed by the rectifying diode 4 and the smoothing capacitor 13, and then output as electric power supplied to a load connected between the high and low voltage output lines 20a and 20b. 
The flyback voltage generated in the feedback winding 2b has a proportional relationship with the flyback voltage generated in the secondary output winding 2c because of the load connected to the output side. As a result of the flyback voltage generated in the feedback winding 2b (refer to the section between time t10 and time t11 of (5) in FIG. 6), the feedback capacitor 12 is charged (refer to the section between time t10 and time t11 of (6) in FIG. 6; in FIG. 4, the bottom electrode is positive, and the top one negative).
At this time, the Zener diode 6 applies a reverse bias to the gate of the FET 3, and acts as a charging current path for charging the feedback capacitor 12 from the low voltage side terminal 1b side.
After the electric energy stored in the secondary output winding 2c caused by the induced counter-electromotive force is discharged (refer to (d) of FIG. 5, and time t11 of FIG. 6), the flyback voltage of the feedback winding 2b that is reverse biased on the gate is reduced (refer to the section between time t11 and time t12 of (5) in FIG. 6). Thus, the gate voltage of the FET 3 exceeds the threshold voltage VTH (refer to (b) of FIG. 5 and time t12 in (2) of FIG. 6) because of the charge voltage that has been held in the feedback capacitor 12 up to this time (refer to (6) of FIG. 6), and the FET 3 is turned on once again. In this way, a series of oscillation operations are repeated.
In this conventional self-excited switching power supply circuit 100, a time constant for the ON-control circuit formed by the feedback capacitor 12 and the feedback resistor 23 is determined. This leads to the feedback capacitor 12 being rapidly charged using the flyback voltage (refer to the section between time t10 and time t11 of (5) in FIG. 6) generated in the feedback winding 2b. 
In other words, the time constant for the ON-control circuit is set such that the feedback capacitor 12 substantially reaches the charge voltage (the flyback voltage) before time t11 when the energy stored in the transformer 2 is discharged from the secondary output winding 2c. Accordingly, when the energy is discharged from the transformer 2 and the flyback voltage is reduced, the FET 3 swiftly moves to the next operation period.
As shown in (d) of FIG. 5, as a result of turning the FET 3 on, the drain (the primary winding 2a side) voltage of the FET 3 changes from roughly 200V (power supply voltage) to 0V at time t12, and then a current begins to flow from the direct current power supply 1.
However, stray capacitance in the windings and parasitic capacitance between the drain and source exist in the primary winding 2a and the FET 3, respectively, and these capacitances are charged by a flyback voltage that makes the bottom side of the primary winding 2a in FIG. 4 a high voltage side during the OFF period of the FET 3. Thus, when the FET 3 is turned on while the drain (the primary winding 2a side) voltage of the FET 3 has not reduced sufficiently, namely, is around 200V, discharge is executed abruptly.
As a result, a large discharge current is generated as shown by A of (c) of FIG. 5, which causes loss of switching devices like the FET 3 to increase, and is a cause of noise.